It's been done for awhile (and I've already made a Wii Sports cover for it) but my USA Wii box template is done.
Get it all at www.lyris-lite.net/covers.html - the template is at the top of the page and Wii Sports is down at the bottom.
It's high res (designed for printing) but you can resize if you want, but I don't recommend it (why throw res away?). Also some things to add, because of difficulties in finding fonts, this is NOT an identical replica of the Wii template but instead a very close (like, 98%) approximation. You might not notice the differences though.